The subject matter disclosed herein relates generally to a system for dissipating regenerative energy in a power converter and, more specifically, to distributing dissipation of regenerative energy among multiple power converters sharing a common DC bus.
Adjustable speed motor drives (ASD) are used to control the speed of AC motors and are a common type of power converter to share a DC bus. AC motors use three-phase electrical power connected to the stator windings of a motor to run the motor. Each stator winding is connected to a different conductor from a three-phase power source, in which each conductor delivers a different phase of the electrical power to the motor. The three-phase power source may be a direct connection to line power, but more commonly, the motor is connected to the ASD. The ASD allows for speed control of the motor not available by connecting the motor directly to line power.
As is known in the art, there are many electrical topologies for ASDs used to convert the fixed voltage and frequency from the line input into a controlled voltage and frequency output for a three-phase motor. One common topology includes a rectifier section which converts the line power into a DC voltage used to charge a DC bus section of the ASD. An inverter section then controls a set of solid state switches, for example, via pulse width modulation (PWM), to convert the DC voltage from the DC bus into a variable voltage and frequency output to the motor. Controlling the variable voltage and frequency output to the motor controls the speed at which the motor rotates.
As the ASD controls the speed of the motor, there are periods of operation when the motor may enter a regenerative condition such as decelerating a high inertial load or maintaining a constant speed in the presence of an overhauling load (i.e., a load that would tend to accelerate the speed of the motor). Under a regenerative operating condition, the motor operates as a generator sending power back through the inverter section and onto the DC bus section of the ASD, causing the voltage level on the DC bus to rise. Unless this power is removed from the DC bus in some manner, the voltage continues to rise until it becomes too great, causing an over voltage fault and shutting down the ASD.
One way to avoid an over voltage fault is to provide the ASD with a conductive path connected to the DC bus on which to shunt the power generated by the motor. It is known to establish this alternate conductive path by selectively connecting an external resistor to the DC bus via an internal, solid-state switch such as a transistor. When the resistor is connected to the DC bus, current flows through the resistor and the power is dissipated from the resistor as heat. Control of the switch is performed as a function of the voltage level present on the DC bus.
In systems in which multiple ASDs are present, it is also known to electrically connect the DC bus of each ASD, which is also referred to as providing a common DC bus for multiple ASDs. With a common or shared DC bus, when a first ASD is operating in a regenerative mode, a second ASD may be operating in a motoring mode. The second drive uses a portion or all of the power regenerated from the first drive to operate the second drive. However, in a shared DC bus system, operating conditions still exist in which more energy is regenerated onto the DC bus than is consumed by the ASDs in the system. As an example, both ASDs may simultaneously operate in a regenerative mode. Thus, an alternate conductive path is still required.
However, sharing a DC bus among multiple ASDs each having a corresponding shunt resistor is not without its drawbacks. If a first ASD is configured to connect its respective shunt resistor at a lower voltage level than a second ASD, the first shunt resistor will be more heavily utilized to dissipate excess energy on the DC bus than the second shunt resistor. Further, shunt resistors are sized, in part, according to the power rating of the ASD to which they are connected. If the second ASD has a higher power rating than the first, the shunt resistor of the first ASD is subject to excessive loading and premature failure. Even if the first and second ASDs are configured to connect their respective shunt resistors at the same voltage level, measurement noise and bias will result in one of the two ASDs connecting its shunt resistor first, resulting in the same potential for excessive loading and/or premature failure of the shunt resistor.